override func viewDidLoad() {
    super.viewDidLoad()
    navigationController!.interactivePopGestureRecognizer?.delegate = self
  }

extension YourViewController: UIGestureRecognizerDelegate {
  func gestureRecognizerShouldBegin(gestureRecognizer: UIGestureRecognizer) -> Bool {
    
    if navigationController?.viewControllers.count == 2 {
      return true
    }
    
    if let navigationController = self.navigationController as? NavigationStack {
      navigationController.showControllers()
    }
    
    return false
  }
}